Electrolyte for electrolytic condenser



United States Patent ELECTROLYTE FOR ELECTROLYTIC CONDENSER denser & Electronics Corporation, Boston, Mass., a

corporation of Massachusetts No Drawing. Application June 20, 1956 Serial No. 592,504

11 Claims. (Cl. 252-622) This invention relates to a novel type of electrolyte for use in electrolytic capacitors and is especially useful in capacitors that are intended to be used in alternating current circuits.

It has been recognized for many years that metals such as aluminum and tantalum can be anodized to provide an extremely thin oxide film thereon having excellent dielectric properties and that metal foils having such films thereon can be advantageously employed in the manufacture of capacitors. Electrolytic capacitors employing filmed electrodes of this type in conjunction with so-called dry electrolytes have been extensively used commercially in direct current circuits.

Theoretically the same principle can be employed in capacitors to be used in AC. circuits for power factor correction and other purposes. However, when such capacitors were introduced into A.C. circuits a number of problems were encountered which have delayed the gen eral use of these capacitors for alternating current applications. The anodized film because of its extreme thinness is readily abraded and can be easily destroyed by a variety of electro-chemical and chemical reactions. Removal of the film from a very small area of the electrode seriously impairs the efficiency of the capacitor. When such capacitors are used in AC. circuits it is evident that during half of each cycle the conditions at the filmed electrode are such as to favor removal of the oxidized film. In practice it was found that such capacitors deteriorate over a period of time and do not possess either a low enough power factor or sufficient electro-chemi'cal irreversibility to be used for continuous duty in alternating current circuits.

Many modifications and variations of these capacitors have been proposed, and in recent years a relatively successful capacitor has been developed which employs an essentially non-aqueous electrolyte comprising a ketone or aldehyde solvent that inhibits progressive deterioration of the oxide film when the capacitor is used in A.C. circuits. Capacitors incorporating such an essentially anhydrous electrolyte have been built and have had power factors of as low as 2% or less and satisfactory stability for continuous duty A.C. service under certain conditions. However, under other conditions, as for example where the capacitor is to be used at an elevated ambient temperature or under such conditions that the capacitor heats up substantially, certain difficulties are encountered which stem largely from the properties of the ketone or aldehyde solvent used. These compounds in general have low flash points and high volatilities. Hence at elevated temperatures, they tend to increase the pressure in the capacitor, thereby increasing the tendency toward vapor leakage therefrom. Moreover, if the capacitor is operated under such conditions that leakage of these vapors 2,886,528 Patented May 12, 1959 "ice and tried, but this substitution was not successful because the use of such higher molecular weight solvents diminished the AC. stability of the capacitor and increases its power factor.

It is accordingly an object of the present invention to provide a capacitor which has a low power factor and satisfactory stability for continuous duty alternating current service and which is at the same time capable of retaining these characteristics over a relatively wide temperature range including both relatively low and relative- 1y high temperatures. It is another object of the invention to provide an electrolyte suitable for use in alternating current capacitors and which is composed entirely of materials of low volatility which either have high flashpoints or are essentially inert toward atmospheric oxidation except under extreme conditions. Other objects of the invention will be in part obvious and in part pointed out hereafter.

In general an electrolyte composition prepared according to the present invention comprises an essentially anhydrous mixture of a relatively weak organic acid, an organic liquid which may be called a solvent and which is preferably selected from the group consisting of polyhydric alcohols and their ethers, and a salt of an organic acid which is capable of being dissolved in and ionizes in the mixture of acid and solvent to provide metallic cations in the electrolyte. The organic acid salt, as well as the other components of the electrolyte, is preferably free from nitrogen. The preferred salts are salts of the alkali metals i.e. sodium, potassium, lithium, etc. The preferred acidic component is phenol per se, although substituted phenols, e.g. cresols, may also be used. When phenol is used the salt can conveniently be an alkali metal phenolate such as sodium phenolate. The salt may be added to the mixture as such, or a quantity of a concentrated aqueous alkali can be added which reacts with the acidic component to form a salt.

The polyhydric alcohol or ether is added to the acid and salt in a sufficient amount to form a fluid mixture. As indicated in the specific examples given below, al-

though the polyhydric alcohol or ether is referred to occurs, an explosion hazard is created because of the of high molecular weight ketones and aldehydes having higher boiling points and flash points has been proposed herein as a solven it need not be present in major proportion in the mixture. Among the solvents that are useful in the present composition are ethylene glycol, diethylene glycol, glycerine and the ethers of these polyhydric alcohols such as the methyl, ethyl, propyl, butyl, amyl and hexyl ethers, that is, ethers wherein the hydrocarbon ether-forming radical contains from 1 to 6 car-' bon atoms.

The proportions of the three constituents of the elec trolyte described above can vary considerably. Thus the organic acid component may vary from below about 1% to more than by weight of the mixture. The salt is preferably present in a relatively small amount, say 0.1 to 5% by weight. More generally the quantity of salt used is such as to provide an electrolyte having a desired conductivity e.g. a specific resistivity of 2000 to 8000 ohmcentimeters. The remainder of the mixture is solvent and the solvent may vary from less than about 5% to more than 99% by weight of the mixture.

In order to point out more fully the nature of the present invention a number of specific examples are given below of compositions prepared according to the present invention. The compositions described in the examples were tested to determine their flash-point and resistivity immediately upon their formation prior to aging. Also the power factor obtained with most of these composi-. tions was determined under standard conditions by'immersing in the composition as an anode an aluminum foil having an oxide film thereon formed by application of a ZOO-volt direct current thereto. The results of these tests are listed in the several examples.

- assasae EXAMPLE 1 Flash Resistivity, Power Composition, Wt. in grams Point, Ohm-ems. Factor, 0. Percent 2 o en0.a e Dlethylene glycol mono- 92 970 7 methyl ether 40 EXAMPLE 2 2 o eno a e Diethylene glycol monoethyl 91 890 7 ether 4O EXAMPLE 3 li i 1i""i Y g o one a e Diethylene gly 91 250 7 butyl ether 4O EXAMPLE 4 Phenol 130 Sod. Phenolate 5 88 3, 050 2. 2 Ethylene glycol 20 EXAMPLE 5 EXAMPLE 6 Phenol 120 Sod. Phenolate 6 3,420 2. 8 Glycerlne 20 EXAMPLE 7 Ortho-Chloro-Phenol 120 Sod. Phenolate 6 3 120 2 7 Dlethglene glycol monomet yl ether 40 7 EXAMPLE 8 Phen 130 Sod. Phenolate 6 2 220 3 Mixed Polyethylene Glycols of about 400 molecular wt-- 20 EXAMPLE 9 2 o eno a e- Ethylene glycol monomethyl 400 7 other 40 EXAMPLE 10 It should be noted that the power factor values given in the foregoing examples were measured shortly after the electrolytes were prepared. The power factors of capacitors containing such electrolytes ordinarily decrease over a period of time in use, and hence in commercial use, power factors lower than those indicated in the examples can be expected.

From the foregoing description it will be apparent that the present invention provides a group of electrolytes which permit the achievement of the several objectives set forth at the beginning of the present specification. The compositions disclosed have resistivities within the desired range of 2000 to 8000 ohm-centimeters and capacitors incorporating these electrolytes have relatively low power factors which they retain for extended periods of time when properly connected in AC. circuits. Moreover such capacitors are useful over an exceptionally wide temperature range. Thus the polyhydric alcohols and their ethers possess anti-freeze properties that permit capacitors containing the present electrolytes to be used at low temperatures of the order of arctic temperatures. On the other hand all components of the electrolyte have low volatilities and hence may be satisfactorily used at relatively high temperatures or under conditions such that the capacitor tends to heat up in use.

It is of course to be understood that the foregoing examples are illustrative only and that numerous changes can be made in the ingredients and proportions disclosed without departing from the spirit of the invention as defined in the appended claims.

This application is a continuation-in-part of my prior application Serial No. 483,815, filed January 24, 1955.

I claim:

1. A substantially anhydrous electrolyte adapted to be incorporated in an electrolytic capacitor for use over a relatively wide temperature range, said electrolyte consisting essentially of an organic solvent which is an ether of a polyhydric alcohol wherein the ether forming radical contains from one to six carbon atoms, and a solute essentially composed of a phenol in major proportion and a smaller amount of an alkali metal phenolate.

2. A substantially anhydrous electrolyte adapted to be incorporated in an electrolytic capacitor for use over a relatively wide temperature range, said electrolyte consisting essentially of a major proportion of an organic solvent which is an ether of a polyhydric alcohol wherein the ether forming radical contains from one to six carbon atoms, and a solute essentially composed of 'a phenol and a relatively small amount of an alkali metal phenolate.

3. A substantially anhydrous electrolyte adapted to be incorporated in an electrolytic capacitor for use over a relatively wide temperature range, said electrolyte being essentially composed of a glycol ether wherein the ether forming radical contains from one to six carbon atoms, a relatively small amount of a phenol, and a smaller amount of an alkali metal phenolate.

4. An electrolyte according to claim 3 and wherein said glycol ether is ethylene glycol monomethyl ether.

5. An electrolyte according to claim 3 and wherein said glycol ether is ethylene glycol monoethyl ether.

6. An electrolyte according to claim 3 and wherein said glycol ether is diethylene glycol monomethyl ether.

7. A substantially anhydrous electrolyte adapted to be incorporated in an electrolytic capacitor for use over a relatively wide temperature range, said electrolyte consisting essentially of about 1% to 45% by weight of a phenol, 0.1 to 5% by weight of i311 alkali metal salt of a weak acid, and the remainder being essentially an ether of a polyhydric alcohol wherein the ether forming radical contains from one to six carbon atoms.

8. A substantially anhydrous electrolyte adapted to be incorporated in an electrolytic capacitor for use over a relatively wide temperature range, said electrolyte being essentially composed of about 1% to 45 by weight of a phenol, 0.1 to 5% by weight of sodium phenolate, and 5 to 99% by weight of a glycol ether wherein the ether forming radical contains from one to six carbon atoms.

9. An electrolyte according to claim 8 and wherein said glycol ether is ethylene glycol monoethyl ether and said phenol is cresol.

10. An electrolyte according to claim 8 and wherein said glycol ether is ethylene glycol monomethyl ether and said phenol is phenol per se.

11. An electrolyte according to claim 8 and wherein said glycol ether is diethylene glycol monomethyl ether.

References Cited in the file of this patent UNITED STATES PATENTS 2,036,669 Yngve Apr. 7, 1936 2,089,684 Clark Aug. 10, 1937 2,164,742 Harding July 4, 1939 2,206,430 Robinson July 2, 1940 FOREIGN PATENTS 387,437 Great Feb. 9, 1933 

1. A SUBSTANTIALLY ANHYDROUS ELECTROLYTE ADAPTED TO BE INCORPORATED IN AN ELECTROLYTIC CAPACITOR FOR USE OVER A RELATIVELY WIDE TEMPERATURE RANGE, SAID ELECTROLYTE CONSISTING ESSENTIALLY OF AN ORGANIC SOLVENT WHICH IS AN ETHER OF A POLYHYDRIC ALCOHOL WHEREIN THE ETHER FORMING RADICAL CONTAINS FROM ONE TO SIX CARBON ATOMS, AND A SOLUTE ESSENTIALLY COMPOSED OF A PHENOL IN MAJOR PROPORTION AND A SMALLER AMOUNT OF AN ALKALI METAL PHENOLATE. 